1. Field of the Invention
The present invention relates to a microcomputer reset device for releasing the reset of a microcomputer when its power supply voltage exceeds its operation lower voltage limit during power-up.
2. Description of Related Art
FIG. 5 is a circuit diagram showing a conventional microcomputer reset device, in which the reference numeral 1 designates a power supply for outputting a supply voltage Vcc; 2 designates a resistor with its first end connected to the power supply 1; 3 designates a capacitor with its first end connected to a second end of the resistor 2, and its second end connected to a ground 4.
Next, the operation of the conventional reset device will be described.
First, to release the reset of the microcomputer, it is necessary to set an appropriate lower voltage limit of the supply voltage to the microcomputer in accordance with the operation frequency of the microcomputer. However, applications of microcomputers are versatile, and their operation frequencies are diversified. Generally, the operation lower voltage limit of the microcomputer increases with the operation frequency.
The conventional microcomputer reset device outputs as a reset signal a divided voltage Vd obtained by dividing the supply voltage Vcc with the resistor 2 and the capacitor 3. The divided voltage Vd rises as shown in FIG. 6 during power-up, producing a reset signal as long as it is lower than a threshold voltage Vset which is set at half the supply voltage Vcc, for example.
Accordingly, the reset of the microcomputer is carried out from time T0 to time T1 in which the divided voltage Vd is less than the threshold voltage Vset. Since the divided voltage Vd exceeds the threshold voltage Vset at time T1, the reset of the microcomputer is released, and the microcomputer starts its normal operation.
However, when the rise of the supply voltage Vcc is slow, the rise of the threshold voltage Vset is also delayed as illustrated in FIG. 7. In this case, it sometimes occurs that the divided voltage Vd exceeds the threshold voltage Vset from the very beginning of the power-up (always Vd  greater than Vset) so that the microcomputer cannot be reset normally.
Taking account of this, there is a reset device as shown in FIG. 8 which can reset a microcomputer normally even if the rise of the supply voltage is slow. In this reset device, a comparator 7 compares a divided voltage Vd with a reference voltage Vref output from a constant voltage generator 6, enables the reset signal RESET for resetting the microcomputer as long as the divided voltage Vd is lower than the reference voltage Vref, and disables the reset signal RESET to release the reset of the microcomputer when the divided voltage Vd exceeds the reference voltage Vref.
In some cases, however, such a reset device can cause a problem in that the microcomputer starts its operation at the supply voltage Vcc at which it cannot operate normally. This is because without adjusting the divided voltage Vd for each application circuit of the microcomputer by suitably controlling the values of the resistors 2 and 5, the divided voltage Vd can exceed the reference voltage Vref before the supply voltage Vcc exceeds the operation lower voltage limit, and hence the reset of the microcomputer is released.
Thus, it is not unlikely for the conventional reset devices with the foregoing configurations to start the operation of the microcomputer before the reset is completed if the rise of the power supply is not quick enough. This presents a problem of being unable to ensure the stable normal operation of the microcomputer.
Although the reset device with the comparator 7 as shown in FIG. 8 can start the operation of the microcomputer after it positively resets the microcomputer even in the case of a slow rising power supply, it has another problem of increasing the manufacturing cost because of design changes required for each application circuit of the microcomputer such as altering the values of the resistors 2 and 5.
The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a microcomputer reset device, which can start the operation of the microcomputer after positively completing its reset without requiring any design changes for individual application circuits of the microcomputer.
According to a first aspect of the present invention, there is provided a microcomputer reset device comprising: an initializing circuit for placing a microcomputer in a reset state as long as a first divided voltage proportional to a supply voltage is lower than a reference voltage, and for releasing the reset state of the microcomputer when the first divided voltage exceeds the reference voltage; an adjusting circuit for adjusting, in response to an operation frequency of the microcomputer, a second divided voltage which is proportional to the supply voltage; and a switching circuit for comparing the second divided voltage adjusted by the adjusting circuit with the reference voltage, and for switching, when the second divided voltage exceeds the reference voltage, a clock source of the CPU from an internal clock signal to an external clock signal.
Here, the microcomputer reset device may further comprise a reset circuit for resetting, after the switching circuit switches the clock source of the microcomputer from the internal clock signal to the external clock signal, the microcomputer if the second divided voltage adjusted by the adjusting circuit drops below the reference voltage with canceling a reset operation of the initializing circuit.
The reset circuit may release the reset of the microcomputer when the second divided voltage exceeds the reference voltage.
According to a second aspect of the present invention, there is provided a microcomputer reset device comprising: an initializing circuit for placing a microcomputer in a reset state as long as a first divided voltage proportional to a supply voltage is lower than a reference voltage, and for releasing the reset state of the microcomputer when the first divided voltage exceeds the reference voltage; an adjusting circuit for adjusting, in response to an operation frequency of the microcomputer, a second divided voltage which is proportional to the supply voltage; a counter for counting an interval from a time when the initializing circuit releases the reset of the microcomputer to a time when the second divided voltage adjusted by the adjusting circuit exceeds the reference voltage; and a switching circuit for calculating a clock switching time from the interval counted by the counter, and for switching, when the clock switching time comes, a clock source of the CPU from an internal clock signal to an external clock signal.